Solvent extraction process for protactinium



' to Pa emission to U The half-livesof Th 3 and Pa are twenty-three minutes and twenty-seven and four-tenths days, respectively. Since U is fissionable, it is desir- United t te Fermi EXTRACTION PROCESS FOR PROTACTINIUM Earl K. Hyde and Leonard I. Katzin, Chicago, and Michael J. Wolf, Peoria, 111., assignors to the United States of America as represented by the United States -Atomic Energy Commission No Drawing. Filed May 12, 1948, Ser. No. 26,72

7 Claims. (Cl. 23-145) SOLVENT This invention relates to the separation of protractinium from an aqueous solution by solvent extraction and more especially relates to the separation of protactinium from a mixture of protactinium'and thorium by a solvent extraction process.

Protactinium occurs in nature as the decay product of uranium Y, a short-lived beta-emitting thorium isotope which in turn is formed from the very long-lived uranium isotope, U found to the extent of 0.7% in natural uranium.

protactinium per million parts of uranium, thisvalue being fixedby the relative decay rates of U and Pa 3 For comparison, radium ispresent at -0.35 part per million of uranium. The formidable task of isolating protactinium occurring in such a minute concentration is somewhat lightened by the availability of waste fractions from the commercial processing of uranium ores which contain slightly greater concentrations of protactinium and which are more amenable to chemical treatment than the raw ores. During the processing of such uranium ores the major portion of the uranium andsome of the other constituents are dissolved in nitric acid. By the addition of sodium carbonate some of the dissolved impurities, which include protactinium, polonium, ionium (Th and radium are precipitated. This sodium carbonate precipitate is available for recovery of protactinium by separation from thorium and the other constituents.

In some of the processes for recovery of protactinium from uranium ores, carrier precipitation techniques were developed using tantalum and zirconium compounds. Using the tantalum carrier technique the product obtained was substantially tantalum and contained notmore than about 0.1% protactinium. In the zirconium carrier technique, it was necessary to separate protactinium from zirconium by fractionalcrystallization of zirconium oxychloride.

With the development of the uranium-graphite pile for production of plutonium, fissionproducts, and energy, it was found that an increased utilizationof neutrons could be accomplished by the use of a blanket of thorium or a thorium-containing material around the pile. Bythe use of this blanket the thorium isotope, Th absorbed neutrons to form Th which decayed by beta-emission This protactinium isotope decayed by beta- Protactinium, therefore, occurs naturally only .in uranium ores, and to the extent of'about 0.25 part 2,978,294 P ten de 4 1 6.1-

process it is necessary to separate protactinium as well as uranium from thorium. This can be accomplished either by first removing U by known processes and then separating Pa from thorium or by separating U and Pa simultaneously from thorium. The' amounts or Pa and U produced by, the neutron-irradiation of thorium are quite' small, rarely beingabove 1% by weight of thorium and usually being'substantially below" this concentration.' Thus, it is necessary'to recover Pa and U from thorium masses having U and Pa concentrations below one part per thousand parts and even one part per million parts of thorium. -In allof the foregoing materials containing protactinium the materials are dissolved by strong inorganic acids and the resultant aqueous solutions usually contain very dilute concentrations of protactinium-salts.

It is an object of this invention to separate protactinium i from aqueous solutions of protactiniumssalts--- It is another object of the present invention to-separate protactinium from mixtures of protactinium and thorium.

.It is still a further objectof the present invention to. separate protactinium and uranium from a mixture, of protactinium, uranium, and thorium. 7

Still another object of this invention is to provide "a process for separating protactinium from afmixture of protactinium and zirconium; V V 7 Other objects of the present invention will be apparent from the description which follows. We havefound that Jprotactinitimlan be separated from an aqueous solution on protactinium salt by providing the aqueous solution with a strong inorganic acid in a con centration of 0.1 to 10 Nfand cummin the aqueous' solution withacertain type of organic isolvent.

More particularly}theprocess of this invention comprises contacting an aqueous solution containing a water soluble ;J rota ctiniun1 salt and a' strong inorganic acid in a concentration of 0.1 to 10Nwith an aliphatic oxygencontaining organic solvent having at least six carbon atoms and selectedfrorri the group consistingjof alcohols, ketones, and este"rs, and mixtures thereof, and separating able to, remove it from the thorium blanket before the concentration ofU becomes very high. Otherwise, there are fission products of U inthe blanket so that -the purificationof thorium for re-use in the blanket cannot be carried out by a simple procedure. This puri-I fication of thorium could be accomplished by removing the'blanketfrom the pile after a certain period of irradia-- I tion and, storing the blanket for a sufficientfperiod of time to allow substantially all of the protactinium present the resultant aqueous phase and organicsolvent extractphase containingprotactinium salt.

"As will be seenhereinafter, it isneces s'ary, when ex- I tracting a protactinium salt from anaqueo'us solution by a. solvent of this invention, that at least 0.1 N; strong inorganic acidbe present in the aqueous phase; otherwis'e, there is no protactinium extraction -orthe'faniount of extraction is negligible. Examples of suitablestrongim organic acidsare nitric acid and hydrochloric acid. is desirable that the concentration of inorganic acid be 10 'N or below, especally in the case of reactive acids}.

such as nitric acid, in order to'reduce the reaction of the acid with an organic solvent of this invention. lt'is preferred that the concentration of strong inorganic-acid befbetWeenOJ and 5 N. This is especially t'r'ue when' it is desired toseparate protactinium from a inixture of: protactinium and thorium using an aqueous solution, since at the higher acid I creased amount of thorium extraction.

The :aliphatic oxygen-containing organic's'olvents of- -'the present inventioh arepreferablyialcohols, ketones, 7 and esters having at least six carbon atoms, since alcohols;

ketories, and esters having" less" thanasix'carbonf:atomsw appear to be" practically .in'eifective tor-thefextraction 10f concentrations there is: an in- V Also, ,low-molecular-weight alcohols, ke-

Examples of protactinium. tones and esters are miscible with water. suitable alcohols, ketones, and esters are:

n-heptanol n-octanol 2'-ethylhexanol heptadecanol (3,9-diethyl-6-tridecanol) methyl isobutyl ketone diisopropyl ketone methyl n-amyl ketone sec-butyl acetate n-butyl acetate n-amyl acetate ammonium nitrate calcium nitrate aluminum nitrate magnesium nitrate manganese nitrate thorium nitrate:

Whena strong inorganic acid other than nitric acid is used, the analogous salts of that acid are preferably used. Of course, when it is desired to separate protactinium from large amounts of thorium, the aqueous solution obtained by dissolving such a mixture will contain a considerable concentration of a thorium salt, such as thorium nitrate. It will be unnecessary in most instances toadd an auxiliary salting-out agent to increase the degree of protactinium extraction. This is especially true when the acid concentration is 1 N or more.

In one embodiment of the present invention protactinium is separated from an aqueous solution containing 0.1, to 10 N strong inorganic acid by contacting with a solvent of the type described above. thatthe contact time be suflicient for an equilibration of the protactinium salt between the aqueous phase and thesolvent, extract phase. After contacting, the phases are separated and the organic extract phase contains protactinium salt. of the type described above may be present.

In a second embodiment of this invention, the process of the first embodiment is carried out and the, protactinium-salt is separatedtfrom' the organic solvent extract phase by contacting the extract phase with water. When the original extraction of protactiniumsalt is from an:

aqueous solution having a high acidity, i.e., N or above, a considerable amount of strong inorganic acid is extracted by the organic solvent. As a result, when the extract phase is contacted with water, the acid is extracted by the water providing a relatively high acidity in the aqueous phase, thereby reducing the degree of ex- It is preferable In this embodiment, a salting-out agent several minutes was sufficient.

solutions can be varied considerably below the b01ling a, form easily extractable froman acidic solution by the organic solvents of this invention. This is particularly to be preferred when it is desired to repeat at least once the complete cycle of this embodiment. Suitable complexing agentsare oxalic acid and its watersoluble salts.

If water alone is used for re-extraction, protactinium extracted by the water becomes hydrolyzed producing a compound, which is nonextractable by these organic solvents. At the present time the chemical nature of the nonextractable protactinium is not known; however, it is only by treatment of the compound with a concentrated solution of a strong inorganic acid, such as nitric acid, that the nonextractable compound is. converted to an extractable protactinium compound, such as protactinium nitrate.

Another embodiment consists only of the extraction of a protactinium saltfrom an organic solvent of the type disclosed above by contacting with water or an aqueous solution of a complexing agent of the type described above.

In a fourth embodiment protactinium is separated from a mixture of protactinium and thorium by extracting a protactinium salt from an aqueous solution containing.

said protactinium salt, a thorium salt, and 0.1 to 10 N strongi'inorganiciacid int-accordance with the process of the first embodiment. When the mixture also contains uranium,.the process separates protactinium and uranium from the mixture.

. In another embodiment protactinium is separated from a mixture of protactinium and zirconium by treating an aqueous solution of their salts and containing 0.1 to 10 N strong inorganic acid in accordance with the process of the first embodiment.

In still a further embodiment protactinium and polonium are separated from a mixture of protactinium, polonium and thorium, such as is obtained as a carbonate precipitate, in the uranium recovery process mentioned above. This embodiment also utilizes the process of the first embodiment.

In all of these embodiments the contact time may be varied widely but it was found that with adequate mixing The temperature of the points of 'the solutions. However, it is preferred to operate at about room temperature. The ratio of aqueous solution to organic solvent in the extraction or re-extraction may be varied widely, for example, between 10:1 and 1:10.

In the solvent extraction experimentsreported below. in Tables I to XI inclusive, the aqueous solutions were prepared from concentrated stock solutions of thorium nitrate, nitric acid, ammonium nitrate and the various Then Pa tracer, as nitrate, was added. Since Pa tractionof protactinium into the aqueous extract phase..

In such a case, it is desirable to contact the organic solvent extract phase with a small amount of water for removal of the, inorganic acid and, after separating the acidic aqueousphase, to contact theorganic solvent extract phase with an additional amount of water and finally to separate the resultant aqueous extract phase containing protactinium. salt. 7 f

. In this embodiment, itzis. preferred, when'greater than micr'o a'moun'ts ofwprotactinium areirefextracted, that the waterused for rte-"extraction; contain a; complexing agent; i

ior 'pr'octactinium;so thamtheAprotactinium.remains in centrifugation.

other salting-out agents. The proper amounts of these, stock solutions and of distilled water were delivered from burets to a IS-ml. calibrated, glass-stoppered centrifuge cone to make 5 ml. of solution of a desired composition.

has a short half-life, the amount of tracer used in the various experiments varied between 10,000 and 300,000

beta countsiper minute at 10% counting yield. Tracer U as uranyl nitrate, in the amount of 5,000 to 10,000 counts per minute, at 52% counting yield was also added to the aqueous solutions used in the experiments reported in Table Ito determine the percentage of uranium extrac' tion. In each experiment an equal volume of the pal' ticular organic solvent was added, thecone was shaken for five minutes, and the layers were cleanly separated by Aliquots of the solvent layer were analyzed for thorium and uranium salts when present in the original aqueous solution and-for protactinium salt,

- To, determine the percentage of thorium extraction,

excess saturated oxalic,=acid in 1 N hydrochloric acid wasradded to. -a 2 ml. aliquot ofthe solvent, layer to PIG. new wat wsta ate- .5 h? Bassinetsex e tise 35978384 is '16 by-filtration-and was washed. It was dissolved with slight The values of Table II Show the necessity of using a heating in an excess of standard ceric sulfate and the strong inorganic acid in the aqueous solution from which excess of ceric sulfate was determined by back-titrata protactiniumsalt is to be extracted. ing with standard ferrous ammonium sulfate to the ferroin end point. L I

To determine the percentage of uranium extraction a aliquot of the Solvent layer was Shaken With-05 Diisopropyl ketone extraction of I N nitric acid solutions ml. of distilled water in a 5-ml. centrifuge cone. 'After ntaining sailing-out agemsand racer romcrinium the separation of aqueous and organic solvent layers the "mate cone was immersed in a Dry Ice-acetone bath to freeze I the aqueous layer. The organic "solvent was poured out Y r of the cone audit wasdiscarded. Thefrozen aqueous layer was warmed to room temperature to liquefy. The aqueous layer was evaporated on a platinum disc and the amount of U on the disc was determined by measuring the alpha disintegration rate in a standard pulse ionization chamber. When the amount of thorium extracted from the initial aqueous solution was more than'2%, it was' necessary to remove it from theaqueous extract of the organic solvent aliquot prior to evaporating the aqueous extract on the platinum disc. This was done by adding a few drops of '6 N hydrofluoric' acid to precipitate thorium tetrafiuoride, evaporating the supernatant solution on a platinum di'sca'nd countingU This separation of thorium reduced the uranium content of the aqueous extract, and a 10% correction wa applied to the percentage of uranium extraction 7 To determine the percentage of protactinium extraction 0.25-m1. to 2-ml. aliquots of the solvent layer were evaporated over water to obtain aqueous solutions of pro- .30 tactinium nitrate. These solutions were evaporated on either a platinum plate or a 1-inch diameter watch glass. The dry samples were counted on a standard Geiger- Miill'er beta counter.

Ammonium Aumtnum Calcium Magnes- Mangan- Percentla nitrate, N nitrate, N nitrate, N ium ninese niextracted I trate, N tratc, N

"TABLE I Extraction '0; 1 N nitric acid solutions, containing 3 N 'ammomum N nitrate. 4V l Pretreated diisopropyl ketone, re; the ketone was equilibrated with uranyl and protacnmum nitrates l N nitric acid solution con taining same magnesium nitrate concentration but no tracer Pa before use. V

I Percent Percent Percent. 7 W I or ni o p g The eifect of various salting-out agents on the degree a of protactinium salt extraction is apparent from Table III, since about 40% protactinium is extracted from 1 N nitric 72 5 acid solution containing tracer protactinium nitrate and 75 nosalting-out; agent. The approximate value of 40%, was obtained by plotting the data of Table II. At the intermediate'l concentrations of salting-out agents the Z-Ethylhexanol. Heptadecanol Methyl isobutyl ketone Diisopropyl ketone Methyl n-amyl ketoue Sec-butyi acetate".-. n-Butyi acetate"; n-Amylacetate...

" 3 maximum extractions.werep btaineda j V atsolu tion N. aluminum nitrate and thorlunrnltrate I I iristtad of3 N ammonium nitrate and 2 N thorium nitrate. PK W f N thoflum nitrate Fromasolutionw Instead 9 l rmm i a i 5 7 solutions containing 2- N aluminum nitrate, tracer protactinium nitrate and varying nitric acid concen- Thedata of Table I show that the organic solvents of g V tranon this invention are eiiective for separating protactinium frorn aqueous solution and forseparating,protactinium' from mixtures of thorium andrprotactinium, as well-as I V I I separating,protactinium".anduranium from mixtures of Nitric percentpa Percent Th thorium, protactinium and uranium. extracted extracted ii 1" RT BLEn r. piisopr opyIjk-etone extraction of-solutions of tracer pro- .tactinium nitrare' and, varying'nitric acid. concentration N acid, N: f 7 p Percent Pa extracted 0,5 qThe value of nitric 391diQQlllCQ8SihjQXlZlflCilOD of y p 8 protactinium is apparent from the foregoing data of Table" .53 1V. When no nitric acid was used,'th'orium nitrate, by g 72 hydrolysis, provided some nitric acid. The presence of :67 this acid and the use of salting-out agents, viz.,- thorium h and aluminum nitrates'yaccount for the"value"ofr6.-5%.- Pa i i V i TABLEVFT j Diisopropyl ketone-extraction ;31 N-"thorium. nitrate solutions containing 1 N nitric acid, tracer protactinium nitrate and varying calicum-nitrate concentration Calcium nitrate, N V Percent Pn, Percent Th extracted extracted The extractiondataloffTable Show that, when it is desired to separate protactinium-irom-amixture of protactinium and thoriumjarid afsaltijrig-out agent is used, the total nitrate concentration should-be preferably 6 normal or less.

1 TABLETVI Diisopropyl'ketone extraction of 2.5..N. thoriumQnitraIe solutions containing] N nitric acid tracerprotactinium nitrate and varying. magnesium nitrate. concentration Percent Pa :PercentfIh Magnesium nitrate, N

. tracte extrac ed TABLE vn- Diisopropyl ketone extraction of 3 N thorium nitrate solutions containing 0.06 g N nitric acid, tracer protactiniunz nitrate and varying magnesium nitrate concentration Diisobropyl ketone extraction of 3 N thorium nitrate solutions having pH of 1.5 and containing tracer protactinium nitrate and varying; magnesium nitrate con= centration Magnesium nitrate, N H PercencPa- .Iiercent-Th extracted extracted A mpa c .QfiIa Y QW jahsi Y QI. indicate in; necessity of at least 011 N concentration ofa str nggin organic acid in the aqueous solution. The desirability of" 8 keeping "row the total nitrate concentration formaximuin efliciency of separating protactinium from admixture with thorium is apparent from these tables:

TABLE IX Diiso'propyli ketone extraction of 2 N thorium nitrate containing'tracer protactinium nitrate and varying nitric acid and. ammonium nitrate concentrations.

Bretreated diisopropyl ketone extraction of I N nitricacid solutions containing tracer protactinium nitrate and varying thorium nitrateand magnesium nitrate concert.-

- trations Thorium Magnesium PercentPa Percent Th nitrate, N nitrate, N extracted extracted I Tables IX and X also illustrate various embodiments. of the process of the present invention.

TABLE XI Z-ethylhexanol extraction of 3 N thorium nitrate solutions containing 1 N nitric acid, tracer protactinium nitrate, and varying calcium nitrate concentration Calcium nitrate, N Percent Pa Peroeut'lh extracted extracted The data in TableXI show that there is also an optimum salting-out agent concentration for'protactinium separa-. tion using another solvent of this invention. Also, it is preferable to keep the salting-out agent concentration at a minimumto keep the thorium extraction as low as possible. t

' In some ofthe experiments reported in'the foregoing tables the percentage of nitric acid extracted by the organic solvent. was determined; In the absence of thorium nitrate or other'salting-out agent 15% nitric acid was extracted from 2 N nitric acid solution and 32% nitric acid was extracted from 10 N nitric acid. However, usingOJS and' l N nitric acid solutions, as high as-' to 80% nitric-acid 'extra'ctions'were' obtained when saltingout agents were present to provide 8 to 10 N total nitrate; At 6 N total nitrate there was between 30 and 50% nitric acid extractiodf'rbififlj and 1 N nitric acid solutions.

'Aqueoiis'solilti'ons eachko'ntai'nin'g a" tracer quantity ofi P0 as poloniu'm chloride, and containing respectively. '3, '6 and' 9' N" hydrochloric acid were prepared. .Each solution was equilibrated withfan equal volumeof diisopropyl' keto'ne; The extraction data'ai presented below inflabl'XlIi" Th'iable' also'present's'th' "'ntage of poloniu'm salrexaaeiea'nomrNnitric acid'sdlution of a tracer amount of vlo' l as poloniumnitrate and 'contain ing-l N manganese nitrate by equilibrium with an equal volume of diisopropyl ketone. ,j I 1 a I r TABLEXIIT" Diisopropyl'keto'ne extraction of'ac'idic solutions of tracer 'palonium salts Hydro Nitric Manganese Percent chlorle acid, N nitrate, acid, N M extracted At concentrations of nitric acid greater than 1 normal, the percentage of polonium extracted would be more than 7%.

Five milliliters of 1 N nitric acid solution containing 1 N manganese nitrate and tracer quantities of zirconium and columbium salts was prepared. The tracers were salts of Zr and Ch. A mixture of these elements was used because Cb" is formed in the disintegration of Zr as follows:

' only 3.8% of the beta activity, i.e., 3.8% of the mixture of zirconium and columbium salts, was extracted by the ketone.

In extraction experiments using equal volumes of diisopropyl ketone and hydrochloric acid solutions of a tracer mixture of salts of zirconium (Zr and columbium Cb the relative inefiectiveness of their extraction by solvents of this invention was demonstrated. The data are reported below in Table XIII.

TABLE XIII Diisoprapyl ketone extraction of acidic solutions of mixed tracer zirconium and columbium chlorides Percent Zr-Cb extracted Hydrochloric acid, N:

In the foregoing examples the extraction of protactinium salts from aqueous solutions of tracer quantities of the salts was described. Conditions for the extraction of macro-concentrations of protactinium salts from solutions containing relatively high concentrations of impurities were found to be somewhat more critical. The salting-out agents, such as manganese and calcium nitrates, result in the formation of colloidal protactinium solutions from which protactinium is not easily extracted.

Best results are obtained by using only nitric'acid. Although water alone will re-extract .protactinium' salts from the organic solvents, the neutral aqueous solution will convert protactinium to a relatively nonextractable state, i.e., it is not possible to extract the protactinium salt from'the aqueous extract phase after adjusting with a strong inorganic acid to 0.1 to 10 N concentration byv contacting with an organic solvent of this invention. The conversion of protactinium to anonextractable state; can be prevented by using a small amount'of a strong inorganic acid in the water.- Of

" course, to permit adequate re-extractionof protactinium salt from thejorganicsolvent, theacid concentratiorf'of normal. A better means of maintaining protactinium in an extractable state is to use an aqueous solution of a complexing agent, such as oxalic acid. A suitable oxalic acid concentration is 0.1 molar. After re-extraction the solution can be treated with an agent for destroying the complexing agent. For example, the eerie salt, .(NHQ Cell-0 upon addition to the aqueous solution will destroy the oxalic' acid. The protactinium salt may be then extracted from the aqueous solution for further purification with respect to thorium in accordance with this invention.

The following experiments illustrate the re-extraction of protactinium salt from the organic solvent. A diisopropyl ketone solution of tracer protactinium nitrate and containing 1.18 equivalents of nitric acidand 0.153 equivalent of thorium nitrate per liter was contacted with an equal volume of 2 N sodium sulfate solution. In another experiment the ratio of 2 N sodium sulfate to diisopropyl ketone solution was 0.1. The distribution coefiicients of protactinium nitrate between the aqueous and ketone phases in these experiments were 30 and 15, respectively. These results indicate that the re-extraction of nitric acid lowers the distribution coefficient. When another portion of this diisopropyl ketone solution of protactinium nitrate was given a preliminary water wash to reduce the nitric acid concentration, the distribution coefficient for protactinium nitrate using 2 N sodium sulfate was 86. I The process of the present invention may be carried out using well-known extraction procedures and apparatus. Thus, the extraction steps may be effected by the use of batch, continuous batch, batch countercurrent or continuous countercurrent methods. In column operation the organic solvent may be either the dispersed phase or the continuous phase.

r The foregoing illustrations and embodiments of this invention are not intended to limit its scope, which is to be limited entirely by the appended claims.

What is claimed is:

1. A process for the separation of protactinium from thorium, which comprises contacting an aqueous solution phase and the ester extract phase containing protactinium' nitrate.

2. A process for the separation of protactinium and thorium, which comprises contacting an aqueous solution containing protactinium nitrate, thorium nitrate, and between 0.1 and 10 normal concentration of nitric acid with an aliphatic oxygen-containing organic solvent having at least six carbon atoms and selected from the group consisting of alcohols, ketones, and esters, and mixtures thereof, and separating the resultant aqueous phase and the organic solvent extract phase containing protactinium nitrate.

3. A process for the separation of protactinium and thorium, which comprises contacting an aqueous solution containing protactinium nitrate, thorium nitrate, calcium nitrate, and between 0;1 and 10 N nitric acid with 2-ethylhexanol, and separating the resultant aqueous. phase .and the alcohol extract phase containing protactinium nitrate.

4. A process for the separation of protactinium and thorium, which comprises contacting an aqueous solution containing protactinium nitrate, thorium nitrate, a saltingout agent, and between 0.1 and 10 N nitric acid with an aliphatic ketone having at, least six carbon atoms, and

separating the resultant aqueous phase and ketone extract 6. The process of claim 4 wherein the salting-out agent ammonium nitrate and the ketone is diisopropyl ketone. l

7; The prqcess of claim 4 whereln th'e saltifig-but'egent p'a per No. 3.12, pp. l 9 7-222 1952), basedon Report is aluminum nitr ate and the ketone is diisopropyl ketone. (DB-3810. Prepared for publication April 30, 1947.

References Cited in the file of.this-patent q 2 of 3 a l d -i l a g t TID 5223 (Pt uaus an m e: ocumen UNI TED STATES N 5 1.), paper No. 6.2,,pp. 259, 261, 264-266 1952), based 2,227,833 Hlxson et a1 Jan. 7 1941 on Report CC-3365 (Feb. 26, 1946), on work carried OTHER REFERENCES out in the winter of 1944-1945 at the University of Hyde and Wolf: AEC Dqcument TID 5223 (Pt. 1 Chicag (CPY 5223 in 

1. A PROCESS FOR THE SEPARATION OF PROTACTINIUM FROM THORIUM, WHICH COMPRISES CONTACTING AN AQUEOUS SOLUTION CONTAINING PROTACTINIUM NITRATE, A SALTING-OUT AGENT, THORIUM NITRATE, AND BETWEEN 0.1 AND 10 N NITRIC ACID WITH N-AMYL ACETATE, AND SEPARATING THE RESULTANT AQUEOUS PHASE AND THE ESTER EXTRACT PHASE CONTAINING PROTACTINIUM NITRATE. 